Molecular imaging is means for forming an image for observing a biological activity at cellular and molecular levels. The advances in molecular imaging technologies of recent years allow various cellular and molecular behaviors in a living body to be observed from the outside (captured as images), resulting in that the molecular imaging becomes important means for a diagnostic technology in a medical area and for research in a life science area.
Magnetic resonance imaging (MRI), positron emission tomography (PET), and optical topography are mainly studied as techniques of the molecular imaging, and widely utilized in the medical area and in the life science area depending on the purposes. Of those, a molecular imaging technology with PET is widely employed for early diagnosis of tumors and exploratory studies on drug candidates, as means for labelling a specific biomolecule by using a probe. Further, a technology of visualizing a cell and a molecule in a living body with a staining compound allows capturing of changes in a cell and a molecule within a living body in a simple manner, and hence has attracted attention as an important technology in analysis of life phenomena, early diagnosis of various diseases, application to the field of drug discovery, and the like. A staining method of visualizing one of a cell and a tissue has been long known, and various staining agents are used depending on the purposes. For example, fluorescence staining agents such as fluorescein are used for microangiography of the retina (see Neurosurgery, 35, p. 930, 1994) and fundoscopy (see Gastroenterology, 127(3), pp. 706-713, 2004). However, there has been a problem that the fluorescence sensitivity is not sufficient, and a stained site cannot clearly detected owing to scattering by a body tissue. Further, there has been a problem that indocyanine green as a cyanine-based dye does not exhibit sufficient fluorescence intensity in many cases in the above-mentioned applications (see SPIE, 2389, pp. 789-797, 1995), and hence needs to be used in a large amount to increase the sensitivity, with a result that an unnecessary portion may be stained. In addition, there has been a problem that the compound has low light fastness, and hence tends to be discolored during observation. Therefore, there is a demand for the development of a novel fluorescent dye having high luminance and satisfactory light fastness.